1. Field of the Invention
In systems for injecting fuel into the combustion chamber of internal combustion engines, component parts such as switching valves and injection nozzles in high-pressure pumps and in the various embodiments of injectors, nozzle holder combinations, or unit injector systems are made to move. Their motion positively displaces a volume which is replenished on the intake side. For the requisite volumetric flow, the pressures and cross sections must be adapted. If the replenishment of fuel is inadequate, the pressure on the intake side drops. If the vapor pressure of the fluid to be pumped fails to be attained, the column of liquid breaks off, causing cavitation bubbles to form. Upon recompression of the fuel to above the vapor pressure, the collapse of the vapor bubbles causes noise.
2. Description of the Prior Art
With present unit injector systems in self-igniting internal combustion engines, mechanically-hydraulically controlled preinjection phases are generated, which contribute on the one hand to reducing the noise of combustion and on the other to minimizing pollutants. In unit injector systems, a distinction can be made among four operating states. A pump piston is moved upward via a restoring spring. The fuel, which is at a constant overpressure, flows out of the low-pressure part of the fuel supply via the inlet bores, which are integrated with the engine block, and via the inlet conduit into the magnet valve chamber. The magnet valve is opened. Via a connecting bore, the fuel reaches the high-pressure chamber.
Upon a rotation of the driving cam, the pump piston moves downward. The magnet valve remains in its open position, and the fuel is forced by the pump piston via the inlet conduit back into the low-pressure part of the fuel supply.
In a third phase of the injection event, an actuator is triggered by the control unit at a specified instant, so that the actuator is pulled into a seat, and the communication between the high-pressure chamber and the low-pressure part is closed. This instant is also known as the “electrical injection onset”. The high fuel pressure in the high-pressure chamber rises continuously as a result of the motion of the pump piston, and as a result a rising pressure is also established at the injection nozzle. Once a nozzle opening pressure is reached, lifting of the nozzle needle occurs, causing fuel to be injected into the combustion chamber. This instant is also called the “actual injection onset”, or the supply onset. Because of the high pumping rate of the pump piston, the pressure continues to rise during the entire injection event. In a concluding operating state, the actuator is turned off again, after which the actuator opens after a slight delay, and the communication between the high-pressure chamber and the low-pressure part is opened again. As the actuator, magnet valves or piezoelectric actuators can for instance be used.
In this transition phase, the peak pressure is reached. After that, the pressure collapses very quickly. When it falls below the nozzle closing pressure, the injection nozzle closes and terminates the injection event. The remaining fuel pumped by the pump element until the apex point of the driving cam is forced into the low-pressure part via the return conduit.
Single-pump systems of the kind described above are intrinsically safe; that is, in the unlikely event of a fault or defect, no more than one uncontrolled injection can occur: If the magnet valve opens, injection cannot occur, since the flows back into the low-pressure part, and a pressure buildup cannot occur. Since the filling of the high-pressure chamber takes place exclusively via the actuator, when the actuator remains constantly in the closed state no fuel can reach the high-pressure chamber. As a rule, unit injector systems are built into the cylinder head and exposed to high temperatures. To keep the temperatures in the unit injector system as low as possible, cooling of the components of the unit injector system is as a rule done by means of fuel, which in turn flows back into the low-pressure part of the fuel injection system.
The total pressure ptot of a flowing medium is composed of a static pressure component pstat and a dynamic pressure component pdyn. Except for pressure losses, caused for instance by friction, the total pressure established is constant. The kinetic pressure, conversely, is proportional to the square of the flow velocity, in accordance with the following equation:       P          d      ⁢                           ⁢      yn        =            ρ      2        ⁢          v      2      
If the fuel in the pump of the unit injector system is accelerated sharply, the static pressure drops. In the process, it can drop below the vapor pressure, resulting in cavitation.
In the motion of the storage piston, both phenomena can occur. The motion of the storage piston leads to a compression of the fuel in the spring holder. This increases the counterpressure of the injection nozzle, which leads to the end of the preinjection phase. In addition, the compression increases the second opening pressure for the subsequent injection. To assure good emissions outcomes, fast opening of the storage piston is indispensable. From an acoustical standpoint, however, the fast opening is not critical, since then the intake side communicates with the element chamber, in which at this instant high pressure still prevails. Upon the return motion of the storage piston, the positively displaced volume must return into the spring holder. This return flow is effected either via a communication with the return, or a communication with the inflow loop. In the process, the fuel passes through a throttle, whose cross section has a certain value. If the throttle is enlarged, a residual pressure can be maintained as a function of the flow cross section. If the positively displaced volumetric flow is greater than the replenishing quantity, then the pressure in the spring holder drops. If when the pressure in the spring holder drops it drops below the vapor pressure, cavitation can occur.
In the return motion of the storage piston at the end of the injection event, the column of liquid above the storage piston is moved in the direction of the element chamber. At this instant, the pressure in the element chamber is already close to the vapor pressure, and as a result a fast return flow can occur. The high flow velocity can lead to undershooting of the vapor pressure so that once again cavitation can be the result.
European Patent Disclosure EP 0 404 916 B1 has a fuel injection nozzle as its subject. The fuel injection nozzle, embodied in particular as a pump-nozzle, includes a nozzle needle, which is urged in the closing direction by a spring. In the fuel injection nozzle, a pressure chamber upstream of the seat of the nozzle needle is in communication with a storage chamber that is defined by a spring-loaded compensation piston. The compensation piston (also called a storage piston), with its storage piston bush, forms a sealing seat. The storage chamber is located downstream of this sealing seat, as viewed from the pressure chamber. The storage piston, which has a cylindrical guide part, is subjected, on its end remote from the storage chamber, to the pressure in a damping chamber that can be filled with fuel, and it has a peg which dips into a plate that defines the damping chamber and has an opening. The cylindrical guide part of the storage piston has a ratio of the diameter to the height of 1:0.1 to 1:0.4; the peg of the storage piston has a variable cross section that dips into the boundary plate, and on its end toward the storage chamber, the storage piston has a guide extension with grooves.